This invention relates generally to fuel cells, and more particularly to a humid stream orifice design that does not become blocked under freezing conditions, as well as methods of fuel cell system start-up under frozen conditions such that blockage due to ice formation is inhibited.
Fuel cells convert a fuel into usable electricity via electrochemical reaction. A significant benefit to such an energy-producing means is that it is achieved without reliance upon combustion as an intermediate step. As such, fuel cells have several environmental advantages over internal combustion engines (ICEs) and related power-generating sources. In a typical fuel cell—such as a proton exchange membrane or polymer electrolyte membrane (in either event, PEM) fuel cell—a pair of catalyzed electrodes are separated by an ion-transmissive medium (such as Nafion™). The electrochemical reaction occurs when a gaseous reducing agent (such as hydrogen, H2) is introduced to and ionized at the anode and then made to pass through the ion-transmissive medium such that it combines with a gaseous oxidizing agent (such as oxygen, O2) that has been introduced through the other electrode (the cathode); this combination of reactants form water as a benign byproduct. The electrons that were liberated in the ionization of the hydrogen proceed in the form of direct current (DC) to the cathode via external circuit that typically includes a load where useful work may be performed. The power generation produced by this flow of DC electricity can be increased by combining numerous such cells to form a fuel cell stack or related assembly that makes up a fuel cell system.
Various fuel cell system operating conditions can lead to a high water content in one or both of the reactant streams. For example, water generated during operation of the fuel cell system may build up in one or both of the anode stream and cathode stream. In certain operating conditions, it is desirable to remove excess moisture to ensure that ice blockage of key flowpaths is avoided in conditions where such water may be exposed to freezing temperatures or related environmental conditions. Removing water from the fuel cell's anode loop is especially difficult as it doesn't have the high volume and velocity gas flow motive force that the cathode loop does as a way to purge any excess water. As such, starting a vehicular fuel cell system that has moisture present in one or both of the reactant fluid streams is hampered under cold ambient conditions if the low temperatures lead to ice or related blockage of the passageways that normally convey reactant to or from the fuel cells or stack. If a flowpath leading to the anode is blocked with ice, the flow of H2 to the stack is prevented, which in turn leads to a failed cold catalytic heating (CCH) event, and the consequent failure of the vehicle to start.
One way to promote heating within a fuel cell after a period of rest is known as cold catalytic stack heating (CCSH). This approach allows the flow of hydrogen from the anode to the cathode as a way to promote heating during fuel cell cold starts. For the cold start to be successful, flow of hydrogen must occur within two seconds of start. If the orifice in the valve is blocked with ice, CCSH will not occur and the cold start is aborted. Supplemental energy devices (including those capable of imparting heat or vibration to at-risk components) may be also employed to reduce the likelihood of ice-related blockage to fuel cell components. Nevertheless, such measures significantly increase the cost and complexity of the overall fuel cell system.